The present invention relates generally to microturbine power generating systems. More specifically, the present invention relates to an ultra low emission catalytic combustor system for a microturbine power generating system.
The United States Electric Power Research Institute (EPRI) which is the uniform research facility for domestic electric utilities, predicts that up to 40% of all new generation could be provided by `distributed generators` by the year 2006. In many parts of the world, the lack of electric infrastructure (transmission and distribution lines) will greatly expedite the commercialization of `distributed generation` technologies since central plants not only cost more per kilowatt, but also must have expensive infrastructure installed to deliver the product to the consumer.
Small, multi-fuel, modular distributed microturbine generation units could help alleviate current afternoon "brownouts" and "blackouts" prevalent in many parts of the world. A simple, single moving part concept would allow for low technical skill maintenance and low overall cost would allow for wide spread purchase in those parts of the world where capital is sparse. In addition, given the United States emphasis on electric deregulation and the world trend in this direction, consumers of electricity would have not only the right to choose the correct method of electric service but also a new cost effective choice from which to chose. U.S. Pat. No. 4,754,607, which is assigned to the assignee of the present invention, discloses a microturbine power generating system suitable for cogeneration applications.
Yet to make these units commercially attractive to consumers, improvements are needed in areas such as increasing fuel-efficiency, reducing size and weight, and lowering thermal signature, noise, maintenance and cost penalties.
Catalytic combustion has long been known to offer potential for providing ultra-low NOx emissions from gas turbines. However for simple-cycle gas turbines the combustor inlet temperature is typically too low for catalytic operation, particularly at part power, and most particularly for smaller, lower-pressure-ratio engines. The addition of a recuperator to the cycle, which recovers turbine waste heat and delivers a higher combustor inlet temperature at all operating conditions, makes catalytic combustion and ultra-low-NOx emissions possible over the full engine duty cycle, even for small engines.
Catalytic combustion is possible only when the combustor inlet temperature exceeds a minimum value which is a function of the catalyst formulation, and which is typically higher than the combustor inlet temperature for simple-cycle gas turbines, or for recuperated engines at startup conditions. Thus a conventional diffusion-flame preheater is required for engine starting, and for accelerating the engine to the speed necessary to obtain an adequate operating temperature. Once this condition has been reached, the preheater can be shut off. At this point a separate fuel delivery system may be used to introduce fuel into the premix duct, where the fuel is evaporated and mixed with the incoming air, and then introduced into the catalyst bed. In order to obtain minimum combustor emissions and to avoid damage to the catalyst bed, complete evaporation of the fuel as well as very high-quality mixing of air and fuel must be achieved within the premix duct. Within the catalyst bed, combustion is initiated by catalytic action near the bed walls. Under appropriate conditions, dual phase combustion occurs which completes the combustion reaction in the gas phase. This process which results in low NOx production as well as very low Co and HC emission over a wide range of engine speeds and loads.
At the high combustor inlet temperatures necessary for catalyst operation, auto-ignition of the fuel within the premix duct is a distinct possibility. However, this must be avoided because it would result in high flame temperatures and thus high NOx production. This problem is particularly acute for diesel fuel, which exhibits an ignition delay period much shorter than natural gas or even jet fuel. Thus the challenge in designing the premixing system lies in obtaining the necessary fuel evaporation and fuel-air mixing quality while avoiding auto-ignition. In addition, for this application compact size is required. This resulted in the design solution of integrating the preheater with the premix duct, which significantly shortens the overall combustor package. Unlike the premixer, however, the preheater must support a stable flame during its operation.
Catalytic combustion can provide extremely low emissions not only of NOx but of other pollutants as well. This is because the catalyst can support combustion at extremely lean fuel-air ratios, resulting in low peak temperatures within the combustor and hence little or no production of thermal NOx. At the same time carbon monoxide (CO) and unburned hydrocarbon (HC) emissions are minimized by reaction of the fuel at the catalyst surface, which can raise the gas temperature to the point where homogeneous gas-phase combustion can occur. Thus a properly designed catalytic combustor can deliver simultaneously low NOx, CO, and HC emissions over the full engine operating range, in contrast to more conventional lean-burn combustors which may suffer from high NOx or high CO at different points in the duty cycle. In addition, stability and acoustic problems often associated with alternative low-NOx combustors are avoided with catalytic combustion, as are the complications of variable geometry. While catalytic combustion is not a new technology, the current example makes use of novel design practices resulting in a compact package suitable for mobile applications.
It has long been known that exhaust gases produced by combusting hydrocarbon fuels can contribute to atmospheric pollution. Exhaust gases typically contain pollutants such as nitric oxide (NO) and nitrogen dioxide (NO.sub.2), which are frequently grouped together as NO.sub.x, unburned hydrocarbons (UHC), carbon monoxide (CO), and particulates, primarily carbon soot. Nitrogen oxides are of particular concern because of their role in forming ground level smog and acid rain and in depleting stratospheric ozone.
The rate of the thermal NO.sub.x production in gas turbine combustors is a function of temperature, pressure, and residence time. Turbine efficiency can be increased by raising the maximum operating temperature, although equipment costs increase sharply due to the need for special materials of construction. Conventionally, hot gas is supplied to the turbine from a precombustor burning a fuel with air to supply hot gas to the turbine at maximum allowable temperature, as limited by mechanical construction. This combustor is at elevated pressure and normally uses a clean gas or liquid fuel.
In acknowledging a need to control atmospheric pollution, a gas turbine engine with a catalytic combustor offers the potential of very low emissions once the system is up to full operating temperature. However, during cold starting of the engine, significant levels of emissions are generated until the catalytic combustor has reached its operating temperature.
It would be desirable, then, to provide an ultra low emission catalytic combustor for a gas turbine engine which would allow cold starting of the engine, with the resulting emissions being no higher than those produced during normal running.